The present invention relates primarily to a pneumatic brake booster of a motor vehicle, this booster comprising a nonreturn valve for discharging an internal overpressure.
A braking system for an automobile generally comprises a pneumatic brake booster which assists the driver during a braking operation.
For that purpose, one type of booster known in the prior art, such as the booster 1 represented in FIG. 1a, comprises an enclosure 2 divided into a front chamber 2A and a rear chamber 2B by a skirt 3 fastened to a piston 21 mounted so that it can slide in an axial direction X. The front chamber 2A can be connected to a vacuum source. The rear chamber 2B can either be connected to the front chamber 2A or be connected to a high-pressure source (for example atmospheric pressure).
When the driver of the automobile activates braking by pressing on the brake pedal, he opens a channel which connects the rear chamber 2B with the atmospheric pressure source and closes a channel which connects the rear chamber 2B with the front chamber 2A. Then the pressure differential between the rear chamber 2B (atmospheric pressure) and the front chamber (at vacuum pressure) exerts a force on the skirt 3, this force being the power assistance which assists the driver when he is braking.
The current standards pertaining to automobile braking systems require that these systems proportion a defined level of deceleration to the vehicles incorporating them in the event of power assistance failure and, in particular, in the event of a lack of vacuum pressure in the front chamber 2A. Now, if the air is not emptied from the front chamber 2A, and if the driver brakes, the air present in this front chamber 2A is compressed progressively as the skirt 3 advances and hence exerts a force on the skirt in the opposite direction to the braking force, thereby reducing braking efficiency.
The patent application with the publication number FR 2 864 501 proposes, in order to empty the air in the event of vacuum circuit failure, a booster 1 of the aforementioned type which additionally comprises shut-off means 8 (see FIGS. 1a and 1b) for placing the front chamber 2A in communication with the free air, these means comprising a non-return valve which is urged into the closed position by the action of atmospheric pressure.
In the event of a fault in the vacuum circuit, the air contained in the front chamber 2A does not remain confined within this chamber when the driver actuates the brake pedal. The movement of the skirt 3 compresses the air contained in the front chamber 2A until it causes the nonreturn valve 8 to open. The air is then expelled from the front chamber 2A by flowing through passage orifices in the nonreturn valve.
However, the latter solution presents a problem concerning the position of this nonreturn valve. The fact is that the nonreturn valve 8 is exposed to the dust which may be present in the engine. As a result of this dust, which may accumulate particularly in the space 20 so as to prevent the closure of the valve 8, this valve 8 may hence remain open and hence cause a permanent leak of the vacuum pressure in the front chamber, reducing the power assistance provided by the booster.